This invention relates generally to the field of gasoline-based internal combustion engines and more specifically to direct injection engines.
Direct injection engines, also called xe2x80x9cin-cylinder injection engines,xe2x80x9d inject fuel directly into the cylinders. Diesel engines commonly use direct injection. Gasoline engines, also called spark ignition engines, normally are port fuel injection engines that inject fuel via a carburetor or a fuel injector into the intake manifold or over the intake values into the cylinders. Recently, direct injection spark ignited (xe2x80x9cDISIxe2x80x9d) engines have been proposed. A DISI engine requires the fuel to be injected at relatively high pressure. Such a DISI engine also requires that the air-fuel ratio be leaner than convention gasoline engines and the air-fuel ratio must remain within tighter margins.
DISI engines operate in a stratified mode or a homogenous mode. When a DISI engine is in the stratified mode, the combustion chambers contain stratified layers with different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture, that is a slightly richer mixture, and subsequent strata contain progressively leaner mixtures. When the engine is in the homogeneous mode, a homogeneous mixture of air and fuel is injected into the combustion chamber. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
When a DISI engine operates in the stratified mode, the fuel is injected late in the compression cycle, usually during a compression stroke. Because of the late injection, a very short time is available for mixing of the air and fuel in the cylinder. Stable combustion is obtained because the rich zone air/fuel mixture near the spark plug is within the combustion limits while the overall air/fuel mixture in the cylinder is leaner than the air/fuel mixture normally used when the engine is in the homogeneous mode. When the engine is in the homogeneous mode, fuel is injected during an intake stroke of the engine. More mixing time is available in the homogenous mode. The stratified mode is more fuel efficient than the homogenous mode.
Direct injection engines are commonly coupled to three-way catalytic converters to reduce CO, HC, and NOx emissions. When operating at air/fuel mixtures lean of stoichiometry, an NOx trap or an NOx catalyst is typically coupled downstream of the three-way catalytic converter to further reduce NOx emissions. The stratified mode may be used for light to medium loads and the homogeneous mode may be used for medium to heavy loads.
Gasoline engines typically collect evaporated fuel vapors from the gasoline tank in a carbon canister, also called a charcoal canister or an evaporative canister. The evaporated fuel vapors are purged from the carbon canister into the intake manifold and burned in the combustion chambers, that is in the cylinders, along with the fuel that is injected via the fuel injectors. Since the air-fuel ratio of the evaporated fuel vapors is not known, the net air-fuel ratio in the cylinder or in a strata in the cylinder is not known. Thus, it is desirable to measure the air-fuel ratio of the evaporated fuel vapors and control the air-fuel ratio injected into the combustion chambers accordingly. Traditionally, the fuel level of the evaporated fuel vapors is measured with an exhaust gas oxygen sensor, such as a switching HEGO sensor or a linear UEGO sensor. When the engine is running lean, the exhaust gas oxygen sensor may not be able to accurately detect the fuel level of the evaporated fuel vapors.
As gasoline engines become more efficient, that is they are run leaner, the exhaust gas sensors become less accurate or stop working. For example, a DISI engine operating in the stratified mode uses a much leaner mixture than when it is operating in the homogenous mode. The evaporated fuel vapors, which need to be burned, contain a homogenous mixture of air and fuel. However, the fuel level in the evaporated fuel vapors varies over time. Thus, traditional engines often shut off the flow of evaporated fuel vapors when the engine is running lean, for example in the stratified mode. Shut off of the flow of the evaporated fuel vapors limits the efficiency of the engine by limiting the time the traditional engine can be operated in the stratified mode.
Further, when a traditional DISI engine running in the stratified mode purges evaporated fuel vapors into the combustion chamber, the strata outside the rich stratified zone may be too lean to support combustion. This may result in large quantities of unburned fuel in the exhaust gas, which may result in an undesirable large exothermic reaction in the catalyst as the catalyst oxidizes the unburned fuel. If the evaporated fuel vapors contain too much fuel, making the mixture too rich, the torque produced by the engine undesirably increases.
An engine controller for a direct injection spark ignited internal combustion engine can operate in a stratified mode where fuel is injected during a compression stroke of the engine and a homogeneous mode where fuels is injected during an intake stroke of the engine. The engine controller adjusts the flow rate of evaporated fuel vapors as a function of the catalyst temperature and the fuel level in the evaporated fuel vapors. The engine controller determines the fuel level in the evaporated fuel vapors as a function of the exhaust gas oxygen senor output.
The improved system and method of controlling a direct injection spark ignition engine is described. The improved system and method may use sensors that are found on existing DISI engines to indirectly measure the fuel level in the evaporated fuel vapors. The improved system and method allows the evaporated fuel vapors to be purged into the intake manifold when the engine is run lean. Thus, the engine operates longer in the more efficient stratified mode resulting in a lower fuel consumption.
The foregoing discussion has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention.